1. Field of the Invention
The present invention relates to magnetic resonance imaging and in particular to a method and an apparatus for making a distinction in a magnetic resonance imaging water-fat image.
2. Description of the Prior Art and Related Subject Matter
Magnetic resonance imaging is an imaging modality using the magnetic resonance phenomena. The principles of the magnetic resonance phenomena are fundamentally as follows. The protons of nuclei that contain singular protons (such as hydrogen nuclei that are widely existing in human bodies) have self-spin motion and behave like small magnets, and the self-spin axes of these small magnets have no certain rules; if an external magnetic field is applied thereto, these small magnets rearrange according to the external magnetic field's magnetic force lines, meaning they arrange in two directions which are parallel to or oppositely parallel to the external magnetic field's magnetic force line. The direction that is parallel to the external magnetic field's magnetic force line is referred to as a positive longitudinal axis, and the direction that is oppositely parallel to the external magnetic field's magnetic force line is referred to as a negative longitudinal axis. The nuclei have only a longitudinal magnetization component, and this longitudinal magnetization component not only has a direction but also an amplitude. Radio frequency (RF) pulses at a specific frequency are used to excite the nuclei in the external magnetic field, such that the self-spin axes of these nuclei deviate from the positive longitudinal axis or the negative longitudinal axis and resonate, which is the magnetic resonance phenomenon. After the self-spin axes of the above excited nuclei have deviated from the positive longitudinal axis or the negative longitudinal axis, the nuclei have horizontal magnetization components.
After the radio frequency pulse transmission has stopped, the excited nuclei emit echo signals, due to gradually releasing the absorbed energy, in the form of electromagnetic waves, with both the phase and energy level thereof restoring to the state before being excited. An image can be reconstructed by further processing (such as the spatial encoding, etc.) the echo signals emitted by the nuclei. The process of the excited nuclei returning to the state before being excited is referred to as the relaxation process, and the time required for restoring to the equilibrium state is referred to as the relaxation time.
Inside a human body, the molecular environments in which the hydrogen nuclei in fat and the hydrogen nuclei in water exist are different, so when the excitation is carried out using the same radio frequency pulses, their resonant frequencies are different and the relaxation times are also different. When signals are acquired at different echo times, the fat tissue and the water show different phases and signal strengths.
The Dixon method is a method for producing a pure water proton image in magnetic resonance imaging, and its basic principles are as follows. Two kinds of echo signals, in phase and out phase, of the water and fat protons are respectively acquired and a pure water proton image is produced by calculating these two kinds of signals with different phases and removing the fat signal therein, thus achieving the object of fat suppression. The drawbacks of the Dixon method are that the influence of the magnetic field inhomogeneity thereon is relatively large, it is easily affected by respiratory movements, and the calculation method thereof is complicated and prone to errors.
In order to simultaneously obtain the separated pure water and pure fat images, an improved three-point Dixon method is widely used, and the principles of this method are as follows. One in-phase (or out-of-phase) image and two out-of-phase (or in-phase) images are obtained simultaneously, the added phase caused by the magnetic field inhomogeneity is produced according to these two out-of-phase (or in-phase) images, a phase correction is carried out on these two out-of-phase (or in-phase) images, and then it is used together with the in-phase (or out-of-phase) image to produce a water and fat image.
There are various particular operation processes of the three-point Dixon method, for example, with the use of one in-phase image and two out-of-phase images are used, a method for reconstructing a water-fat separation image is disclosed in Chinese patent application 200510008973.0, that includes the steps of acquiring one in-phase image and two out-of-phase images, obtaining the distribution of the data coil sensitivity of each channel, synthesizing an image of each channel, solving the phase difference of the two out-of-phase images, detecting some feature areas in the in-phase image to be used as criteria for the phase correction, and correcting the phases of the out-of-phase images and calculating images of water and fat.
In addition, a method for implementing water and fat separation is disclosed in Chinese patent application 200910119608.5, that includes the steps of acquiring an in-phase image and two out-of-phase images, calculating the phase difference of said two out-of-phase images, and correcting the phase caused by the magnetic field inhomogeneity in the two out-of-phase images using this phase difference, correcting the linear phase difference caused by the eddy current in the in-phase image by using the two corrected out-of-phase images, and obtaining, by the calculation, a water image and a fat image according to the three corrected images.
FIG. 1 shows a schematic diagram for acquiring data and encoding sequences of two out-of-phase images and one in-phase image with a three-point Dixon method which is based on a turbo spin echo (TSE) sequence. In this case, RF, RO and PE correspond to a radio frequency pulse, a read out gradient and an encoding gradient, respectively.
As shown in FIG. 1, magnetic resonance imaging equipment first transmits a 90 degree radio frequency pulse RF_0, and then transmits again a 180 degree reunion phase radio frequency pulse RF_1. After an echo time (TE) from the 90 degree radio frequency pulse RF_0, the magnetic resonance imaging equipment applies a read out gradient in the read out gradient direction and reads out three echoes 1, 2, and 3, respectively. Subsequently, a 180 degree reunion phase radio frequency pulse RF_2 is transmitted again and the read out gradient is applied in the read out gradient direction, with three echoes 4, 5 and 6 being read out respectively; and the above operation can be repeated many more times. The sequences in FIG. 1 are continuously repeated after the phase encoding gradient has been changed until all the echoes in the k space are read out. In this case, the data lines corresponding to echoes 1, 4, 7, . . . and so on constitute original data of an out-of-phase image, and the data lines corresponding to echoes 2, 5, 8, . . . and so on constitute original data of an in-phase image, and the data lines corresponding to echoes 3, 6, 9, . . . and so on constitute original data of another out-of-phase image. Data in different rows in the k space are obtained by appropriately modulating these echoes using the phase encoding gradient applied in the phase encoding direction, and when the k space is filled up, the corresponding one in-phase image and two out-of-phase images are constituted after having carried out Fourier transform.
However, the defects in the currently available three-point Dixon method are as follows: since the added phase solved using the two, out-of-phase (in-phase) images cannot be directly used to correct their phases, the anti-wrapping has to be carried out on the phases, and due to the inherent instability of phase anti-wrapping, sometimes the calculated images of water and fat may be shifted, i.e. it is actually the image of water, but the image which has been worked out appears to be the image of fat, or it is actually the image of fat, but the image which has been worked out appears to be the image of water.
Aiming at such a water-fat image shift problem, currently there is not yet a solution corresponding thereto, and to this, how to determine whether a current water (fat) image calculated in the magnetic resonance imaging equipment is actually the image of water or that of fat has already become an urgent problem to be solved in magnetic resonance imaging technology.